This invention relates to an improved process for the production of tertiary-butyl hydroperoxide. More particularly, this invention is directed to an improvement in the preparation of tertiary-butyl hydroperoxide by direct, non-catalytic oxidation of isobutane in a reaction mixture at a pressure above the critical pressure of the mixture and at a temperature in the range of about 140.degree. to 170.degree. C., wherein the selectivity with which isobutane is converted to tertiary-butyl hydroperoxide is optimized by controlling the oxygen concentration in the peroxidation reaction zone at critically low levels.
Tertiary-butyl hydroperoxide (hereinafter sometimes referred to as TBHP) is a material of commerce having application as a catalyst, as an initiator for free radial-type reactions and as a starting material or intermediate in the production of valuable chemicals such as oxirane compounds and other organic hydroperoxides.
Because of the ready availability and low cost of starting materials, significant effort has been focused in the past specifically on the preparation of TBHP by direct oxidation of isobutane. Previous disclosures in this area of technology have recognized that the reaction between isobutane and molecular oxygen is inherently nonselective in that significant amounts of tertiary-butyl alcohol and minor amounts of other oxygenated compounds such as acids, aldehydes, ketones and other alcohols are formed in addition to the desired TBHP. According to the work of Winkler et al. (U.S. Pat. No. 2,845,461 and also "Liquid Phase Oxidation of Isobutane", Industrial and Engineering Chemistry, vol. 53 (August, 1961), page 655) the formation of by-products other than the desired TBHP is promoted by the presence of substantial amounts of isobutane in the vapor phase during the course of the oxidation reaction. The oxidation of isobutane had therefore been conducted in the vapor phase in the presence of a catalyst, particularly hydrogen bromide, at lower rate to produce a mixture containing unacceptably large quantities of by-products, and contaminated with catalyst derivatives, e.g., organic bromides. It is taught by Winkler et al that a reaction product consisting essentially of TBHP and tertiary-butyl alcohol can be obtained in high yield by reacting isobutane with molecular oxygen in the liquid phase of a two phase (vapor and liquid) mixture at a temperature of from about 100.degree. to 150.degree. and a pressure of at least 400 psig (up to 700 psig) provided the reaction is carried out in a reaction medium in which the presence of any substantial amount of metal ions is excluded and wherein at least a substantial part of the isobutane is in the liquid phase. Oxidation of isobutane in the liquid phase, generally according to the teachings of Winkler et al, is believed to be a current standard for practice in the art.
In spite of its commercial acceptance, the oxidation of isobutane in the liquid phase remains an inefficient method for preparation of TBHP. For instnace, the reaction is one of low rate, generally requiring a reaction time of several hours. Furthermore, it is recognized that under any given reaction conditions there inherently exists an inverse relationship between isobutane conversion and TBHP selectivity, so that an increase in one is associated with a decrease in the other.
The art contains a number of disclosures of methods for improving some aspect of the overall liquid phase oxidation process. Winkler et al. teach that the oxidation reaction rate in the liquid phase can be enhanced by carrying out the oxidation above the critical temperature of isobutane (134.degree. C.). However, for practice at such elevated temperatures it is necessary that the reaction be conducted in a liquid medium based upon a relatively high boiling point solvent. At temperatures above the critical temperature of isobutane, but below the critical temperature of the reaction mixture, a liquid-phase oxidation can be accomplished. The use of externally-supplied reaction solvents, e.g., organic acids, is discouraged by Winkler et al as tending to increase the complexity of the oxidation reaction and subsequent product separation and recovery. Winkler et al instead propose oxidation above 134.degree. C. in a liquid mixture in which the reaction products, principally tertiary-butyl alcohol and TBHP, act as solvent. However, in comparison with lower temperature liquid-phase oxidation, such practice is said to adversely influence the yield of TBHP. Moreover, it is taught that action in the liquid phase, at a temperature above 134.degree. C., and without an external supply of solvent, requires that the liquid reaction mixture have a composition corresponding to a conversion of isobutane of at least 20% and preferably of more than 30%. Because of the recognized inverse relationship between isobutane conversion and selectivity to TBHP, isobutane oxidation at a temperature above 134.degree. C. according to these teachings is a process in which selectivity to TBHP is inherently limited. Furthermore, enhanced rate in such a process is in large degree the result of the high conversions achieved--since TBHP is itself an initiator for isobutane oxidation, the rate of conversion increases as the concentration of TBHP in the reaction mixture increases. On the whole, the liquid phase oxidation reaction of Winkler et al., even above the 134.degree. C. critical temperature of isobutane, remains a process characterized by a relatively slow rate of reaction and a low selectivity for TBHP.
The relevant art also provides disclosure of more recent work relating to improving the conversion or selectivity of non-catalytic liquid-phase isobutane oxidation. For instance, U.S. Pat. No. 3,478,108 to Grane describes the effects of the addition of minor amounts of water (up to 6 percent) upon the conversion and selectivity in the liquid phase oxidation of isobutane with molecular oxygen to afford TBHP and tertiary-butyl alcohol in accordance with the reaction conditions generally described by Winkler et al.
Further, U.S. Pat. No. 3,907,902 to Grane discloses that the selectivity with which isobutane is converted to TBHP in the direct oxidation reaction can be enhanced by the addition of small amounts of certain alcohols (isopropyl alcohol) to the oxidation reaction zone wherein molecular oxygen is reacted with isobutane, again according to the general conditions described by Winkler et al. Practice according to such methods, however, may prove undesirable, for like the use of externally-supplied reaction solvents disclosed by Winkler et al. addition of foreign substances to the reaction mixture may have adverse effect upon the complexity of the reaction and the subsequent product recovery. Furthermore, the greatest benefits of these processes, with respect to enhanced selectivity, for instance, are realized at relatively low temperature, e.g., about 134.degree. C. or less.
In the commonly assigned, copending application of E. G. Foster and E. F. Lutz, entitled Oxidation of Isobutane under Supercritical Conditions, Ser. No. 308,631, filed on even date herewith, it is disclosed that the production of TBHP per unit of reactor volume per unit of time can be substantially enhanced by conducting isobutane oxidation at least in part in a supercritical reaction mixture at a reaction temperature above the critical temperature of the mixture, at a reaction pressure above the critical pressure of the mixture, and under conditions relating to composition of the mixture.